506 VERTEBRATE LIFE AND ORGANIZATION 



and its effectiveness in insulation. They also depress the skin between 

 the hairs, leaving little hillocks where the hairs emerge. We are familiar 

 with this as "goose flesh." There have been many modifications of hair, 

 e.g., the tactile whiskers of a cat and the quills of a porcupine. Even 

 the "horn" of a rhinoceros, which lacks a core of bone, appears to be a 

 clinnp of specialized hairs. 



Other horny derivatives of the integument include claws, which 

 first appear in reptiles and may be modified as nails or hoofs in certain 

 mammals, the whalebone plates of toothless whales, and the covering 

 of the horns of sheep and cattle. 



Individual mucus-secreting cells are common in the epidermis of 

 fishes, and multicellular mucous glands are abundant in amphibian 

 skin. Fishes and amphibians also have a few cutaneous poison glands. 

 Reptiles have lost the mucous and poison glands, and only a few glands, 

 chiefly scent glands, are present in their dry, horny skin. This paucity 

 persists in birds, but glands have again become abundant in mammalian 

 skin. Alveolar-shaped sebaceous glands, epithelial outgrowths from the 

 hair follicles (Fig. 25.1), discharge their oily secretions onto the hairs. 

 Coiled, tubular sweat glands are also abundant in many areas of mam- 

 malian skin. A little urea and some salts are eliminated in the sweat, 

 but sweat glands are particularly important in secreting water whose 

 evaporation cools the body surface. The vascular supply to the skin, the 

 hairs and their muscles, and the sweat glands all play a role in regulating 

 body temperature. Though the nature and function of their secretion 

 is entirely different, mammary glands are regarded as modified sweat 

 glands. Musk and other scent glands, serving for sexual recognition, are 

 also common in many mammals, although they do not occur in man. 



In lower vertebrates, e.g., in the frog, pigments are contained within 

 chromatophores located beneath the epidermis, and skin color can 

 change by the concentration or dispersion of pigment within these 

 stellate cells. Chromatophores are rare in mammals, but the brownish 

 pigment melanin is present within and between the cells of the epi- 

 dermis. Some melanin is present in the skin of all men (except albinos, 

 p. 683) but it is especially abundant in the skin of Negroes. Skin color is 

 determined not only by the pigment present but by the vascularity of 

 the dermis and by the presence of refractive substances such as guanine. 



215. The Skeleton 



Nature and Parts of the Vertebrate Skeleton. Organisms must re- 

 main small and slow moving unless they have a skeleton for support 

 and to serve as levers on which muscles can act. All vertebrates have a 

 skeleton that provides for this, and encloses and protects some of the 

 more delicate internal organs. The central cavities of the bones of 

 higher vertebrates, which contain red bone marrow, are the sites of the 

 formation of red blood cells and certain of the white cells. The verte- 

 brate skeleton is basically an internal skeleton, for it develops within the 

 skin or in deeper body tissues. None of it is a secretion on the body sur- 

 face, as is the exoskeleton of certain invertebrates, although such struc- 



